Question re SSI benefits?

My youngest daughter gets SSI because she is developmentally disabled. She used to get the full amount but it was recently cut in half because our eldest daughter reached 18. She is a full time student living at home and not working, therefore the family size and expenses are unchanged. This has caused financial hardship. However, our local SSA office says that the cut is correct. However according to what I can find on the SSA website, because our eldest daughter is under 21, in full time education and living at home, she should still be included towards the mount of benefit my youngest daughter receives.